Make numpy optional dependency for torch.cuda.amp (#48154)
Summary: Pull Request resolved: https://github.com/pytorch/pytorch/pull/48154
Test Plan:
Uninstall `numpy` and try to importing `torch`
Discovered while working on https://github.com/pytorch/pytorch/issues/48145
Reviewed By: walterddr
Differential Revision: D25046307
Pulled By: malfet
fbshipit-source-id: c1171a49e03bdc40e8dc1d65928c6c12626e33db
